Dynamic brake circuit assembly for a wind turbine

ABSTRACT

A power converter assembly for an electrical power system connected to a power grid includes a rotor-side converter configured for coupling to a generator rotor of a generator of the electrical power system, a line-side converter electrically coupled to rotor-side converter via a DC link, and a dynamic brake assembly electrically coupled to the DC link. The line-side converter is configured for coupling to the power grid. The dynamic brake assembly includes a plurality of switching devices connected in parallel and a plurality of inductors electrically coupled between the plurality of switching devices.

FIELD

The present disclosure relates generally to wind turbines and, moreparticularly, to improved dynamic brake circuit assemblies for windturbines.

BACKGROUND

Wind power is considered one of the cleanest, most environmentallyfriendly energy sources presently available, and wind turbines havegained increased attention in this regard. A modern wind turbinetypically includes a tower, generator, gearbox, nacelle, and one or morerotor blades. The rotor blades capture kinetic energy of wind usingknown airfoil principles. For example, rotor blades typically have thecross-sectional profile of an airfoil such that, during operation, airflows over the blade producing a pressure difference between the sides.Consequently, a lift force, which is directed from a pressure sidetowards a suction side, acts on the blade. The lift force generatestorque on the main rotor shaft, which is geared to a generator forproducing electricity.

During operation, wind impacts the rotor blades and the blades transformwind energy into a mechanical rotational torque that rotatably drives alow-speed shaft. The low-speed shaft is configured to drive the gearboxthat subsequently steps up the low rotational speed of the low-speedshaft to drive a high-speed shaft at an increased rotational speed. Thehigh-speed shaft is generally rotatably coupled to a generator so as torotatably drive a generator rotor. As such, a rotating magnetic fieldmay be induced by the generator rotor and a voltage may be inducedwithin a generator stator that is magnetically coupled to the generatorrotor. The associated electrical power can be transmitted to a maintransformer that is typically connected to a power grid via a gridbreaker. Thus, the main transformer steps up the voltage amplitude ofthe electrical power such that the transformed electrical power may befurther transmitted to the power grid.

In many wind turbines, the generator may be electrically coupled to abi-directional power converter that includes a rotor-side converterjoined to a line-side converter via a regulated DC link. Further, windturbine power systems may include a variety of generator types,including but not limited to a doubly-fed induction generator (DFIG).

DFIG operation is typically characterized in that the rotor circuit issupplied with current from a current-regulated power converter. As such,the power converter can provide nearly instantaneous regulation of itsoutput currents with respect to the grid frequency. Under steadyoperating conditions, the rotor-side converter controls the magnitudeand phase of currents in the rotor circuit to achieve desired values ofelectromagnetic torque. Reactive power flow into the line-connectedstator terminals of the generator can also be controlled.

Such DFIG wind turbines may or may not be equipped with a dynamic brakethat includes parallel insulated-gate bipolar transistors (IGBTs) whichfeed power into a resistor. Minimum components for the dynamic braketypically include a switch (typically a semiconductor such as an IGBT)and a resistor and may also include one or more diode(s) in parallelwith either the switch, the resistor, or both, as well as othercomponents. Without dynamic braking, typical operation of a DFIG windturbine is configured to regulate the positive sequence voltage with aclosed-loop current regulation scheme which minimizes negative sequencecurrent. As the length of the transmission line feeder to the DFIG windturbine is increased, however, response to grid transients and griddisturbances causes oscillations of power into and out of the powerconverter, which can create disturbances on the DC bus voltage therein.As longer transmission line length is typically desired (and possiblycoupled with larger grid voltage transients), the voltage overshoots onthe DC bus voltage in the power converter may reach a level that damagesthe converter components. Thus, the dynamic brake may be used to controlthe peak voltage on the DC bus.

For conventional dynamic brakes, controls for the switch may be operatedbased solely on the level of the DC bus voltage in the power converter.As converter power levels continue to increase, additional IGBTs must beplaced in parallel to conduct the current. Therefore, it is important tobalance the loss in the parallel IGBTs because the loss directly impactsthe junction temperature, and the IGBT with the highest junctiontemperature is the limit in the total energy that can be fed into theresistor.

Thus, the present disclosure is directed to an improved dynamic brakecircuit assembly for a wind turbine that addresses the aforementionedissues.

BRIEF DESCRIPTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one aspect, the present subject matter is directed to a powerconverter assembly for an electrical power system connected to a powergrid. The power converter assembly includes a rotor-side converterconfigured for coupling to a generator rotor of a generator of theelectrical power system, a line-side converter electrically coupled torotor-side converter via a DC link, at least one sensor configured tomonitor at voltage parameter of the DC link, and a dynamic brakeassembly electrically coupled to the DC link. The line-side converter isconfigured for coupling to the power grid. The dynamic brake assemblyincludes a plurality of switching devices connected in parallel and aplurality of inductors electrically coupled between the plurality ofswitching devices. Thus, when the voltage parameter is at or above avoltage threshold, the dynamic brake assembly is configured to turn onsuch that the plurality of inductors receives at least part of a loadgenerated by the power converter assembly.

In one embodiment, the dynamic brake assembly may include at least oneresistor electrically coupled to a node positioned between the pluralityof inductors. In another embodiment, the resistor(s) may include a splitresistor.

In further embodiments, the plurality of switching devices may bearranged in a plurality of pairs of switching devices connected inparallel. In such embodiments, each of the plurality of inductors may beconnected to nodes between first and second switching devices of each ofthe plurality of pairs of switching devices.

In additional embodiments, each of the plurality of inductors may beconnected in parallel with the resistor(s). In alternative embodiments,the dynamic brake assembly may include a plurality of resistors. In suchembodiments, each of the plurality of resistors may be connected inseries with one of the plurality of inductors between the plurality ofswitching devices to form a plurality of dynamic brake circuits.

In several embodiments, the dynamic brake assembly may further includeat least one snubber capacitor electrically coupled between theplurality of dynamic brake circuits. In such embodiments, the dynamicbrake assembly may include at least one additional resistor connected inseries with the snubber capacitor. In another embodiment, the dynamicbrake assembly may include at least one additional resistor connected inparallel with the snubber capacitor.

In certain embodiments, the plurality of switching devices may beinsulated-gate bipolar transistors (IGBTs). In additional embodiments,the electrical power system may be part of a wind turbine power system.In another embodiment, the generator may be a doubly-fed inductiongenerator (DFIG).

In another aspect, the present disclosure is directed to a powerconverter assembly for an electrical power system connected to a powergrid. The power converter assembly includes a rotor-side converterconfigured for coupling to a generator rotor of a generator of theelectrical power system, a line-side converter electrically coupled torotor-side converter via a DC link, at least one sensor configured tomonitor at voltage parameter of the DC link, and a dynamic brakeassembly electrically coupled to the DC link. The line-side converter isconfigured for coupling to the power grid. The dynamic brake assemblyincludes a plurality of switching devices connected in parallel and atleast one resistance-inductance component electrically coupled betweenthe plurality of switching devices. Thus, when the voltage parameter isat or above a voltage threshold, the dynamic brake assembly isconfigured to turn on such that the resistance-inductance componentreceives at least part of a load generated by the power converterassembly. It should be understood that the power converter may furtherinclude any of the additional features as described herein.

In yet another aspect, the present disclosure is directed to a methodfor controlling peak voltage of a DC link of a power converter of anelectrical power system connected to a power grid with minimal switchinglosses. The method includes electrically coupling a dynamic brakeassembly to a DC link of the power converter. The dynamic brake includesa plurality of switching devices connected in parallel and a pluralityof inductors electrically coupled between the plurality of switchingdevices. The method also includes receiving a voltage measurement of theDC link of the power converter. Further, the method includes comparingthe voltage measurement of the DC link to a voltage threshold. When thevoltage measurement is at or above the voltage threshold, the methodincludes turning on the dynamic brake assembly of the power convertersuch that the at least one inductor receives at least part of a loadgenerated by the power converter.

In one embodiment, the method may include applying hysteresis to thevoltage measurement. In another embodiment, the step of turning on thedynamic brake assembly of the power converter may include determining atleast one gating command for each of the plurality of switching devices.More specifically, in certain embodiments, the step of determining thegating command(s) for each of the plurality of switching devices mayinclude time-shifting on-delays and off-delays of the plurality ofswitching devices to optimize sharing of a load between the plurality ofswitching devices.

In further embodiments, as mentioned, the dynamic brake assembly mayinclude a plurality of resistors and a plurality of inductors coupledbetween the plurality of switching devices, with each of the pluralityof resistors connected in series with one of the plurality of inductorsto form a plurality of dynamic brake circuits. In such embodiments, thedynamic brake assembly may also include at least one snubber capacitorcoupled between the plurality of dynamic brake circuits. Thus, inparticular embodiments, the step of determining the gating command(s)for each of the plurality of switching devices may includesimultaneously turning on the plurality of switching devices to optimizesharing of a load between the plurality of switching devices andtime-shifting turn-off commands for the plurality of switching devicessuch that each switching device turns off at a different time. It shouldbe understood that the method may further include any of the additionalsteps and/or features as described herein.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 illustrates a perspective view of a portion of one embodiment ofa wind turbine according to the present disclosure;

FIG. 2 illustrates a schematic view of one embodiment of an electricalpower system suitable for use with the wind turbine shown in FIG. 2;

FIG. 3 illustrates a block diagram of one embodiment of a controllersuitable for use with the wind turbine shown in FIG. 2;

FIG. 4 illustrates a schematic diagram of one embodiment of a dynamicbrake assembly according to the present disclosure;

FIG. 5 illustrates a schematic diagram of another embodiment of adynamic brake assembly according to the present disclosure;

FIG. 6 illustrates a schematic diagram of yet another embodiment of adynamic brake assembly according to the present disclosure;

FIG. 7 illustrates a schematic diagram of still another embodiment of adynamic brake assembly according to the present disclosure;

FIG. 8 illustrates a schematic diagram of a further embodiment of adynamic brake assembly according to the present disclosure;

FIG. 9 illustrates a schematic diagram of another embodiment of adynamic brake assembly according to the present disclosure;

FIG. 10 illustrates a schematic diagram of yet another embodiment of adynamic brake assembly according to the present disclosure;

FIG. 11 illustrates a schematic diagram of still another embodiment of adynamic brake assembly according to the present disclosure;

FIG. 12 illustrates a flow diagram of one embodiment of a method forcontrolling peak voltage of a DC link of a power converter of anelectrical power system connected to a power grid with minimal switchinglosses according to the present disclosure;

FIG. 13 illustrates a simplified, schematic diagram of one embodiment ofa control scheme for a dynamic brake assembly according to the presentdisclosure; and

FIG. 14 illustrates a detailed, schematic diagram of another embodimentof a control scheme for a dynamic brake assembly according to thepresent disclosure.

DETAILED DESCRIPTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

Referring now to the drawings, FIG. 1 illustrates a perspective view ofa portion of one embodiment of a wind turbine 100 according to thepresent disclosure that is configured to implement the method asdescribed herein. As shown, the wind turbine 100 includes a nacelle 102that typically houses a generator (not shown). The nacelle 102 ismounted on a tower 104 having any suitable height that facilitatesoperation of wind turbine 100 as described herein. The wind turbine 100also includes a rotor 106 that includes three blades 108 attached to arotating hub 110. Alternatively, the wind turbine 100 may include anynumber of blades 108 that facilitates operation of the wind turbine 100as described herein.

Referring to FIG. 2, a schematic view of one embodiment of an electricalpower system 200 that may be used with the wind turbine 100 isillustrated. It should be understood that FIG. 2 is provided as anexample embodiment only and is not meant to be limiting. Morespecifically, as shown, the electrical power system corresponds to adoubly-fed induction generator (DFIG) power system. In alternativeembodiments, however, the electrical power system 200 may correspond toa full power conversion system.

During operation, wind impacts the rotor blades 108 and the blades 108transform wind energy into a mechanical rotational torque that rotatablydrives a low-speed shaft 112 via the hub 110. The low-speed shaft 112 isconfigured to drive a gearbox 114 that subsequently steps up the lowrotational speed of the low-speed shaft 112 to drive a high-speed shaft116 at an increased rotational speed. The high-speed shaft 116 isgenerally rotatably coupled to a generator 118 so as to rotatably drivea generator rotor 122. In one embodiment, the generator 118 may be awound rotor, three-phase, DFIG that includes a generator stator 120magnetically coupled to a generator rotor 122. As such, a rotatingmagnetic field may be induced by the generator rotor 122 and a voltagemay be induced within a generator stator 120 that is magneticallycoupled to the generator rotor 122. In one embodiment, the generator 118is configured to convert the rotational mechanical energy to asinusoidal, three-phase alternating current (AC) electrical energysignal in the generator stator 120. The associated electrical power canbe transmitted to a main transformer 234 via a stator bus 208, a statorsynchronizing switch 206, a system bus 216, a main transformer circuitbreaker 214, and a generator-side bus 236. The main transformer 234steps up the voltage amplitude of the electrical power such that thetransformed electrical power may be further transmitted to a grid via agrid circuit breaker 238, a breaker-side bus 240, and a grid bus 242.

In addition, the electrical power system 200 may include a wind turbinecontroller 202 configured to control any of the components of the windturbine 100 and/or implement the method steps as described herein. Forexample, as shown particularly in FIG. 3, the controller 202 may includeone or more processor(s) 204 and associated memory device(s) 207configured to perform a variety of computer-implemented functions (e.g.,performing the methods, steps, calculations and the like and storingrelevant data as disclosed herein). Additionally, the controller 202 mayalso include a communications module 209 to facilitate communicationsbetween the controller 202 and the various components of the windturbine 100, e.g. any of the components of FIG. 2. Further, thecommunications module 209 may include a sensor interface 211 (e.g., oneor more analog-to-digital converters) to permit signals transmitted fromone or more sensors to be converted into signals that can be understoodand processed by the processors 204. It should be appreciated that thesensors (e.g. sensors 252, 254, 256, 258) may be communicatively coupledto the communications module 209 using any suitable means. For example,as shown in FIG. 3, the sensors 252, 254, 256, 258 may be coupled to thesensor interface 211 via a wired connection. However, in otherembodiments, the sensors 252, 254, 256, 258 may be coupled to the sensorinterface 211 via a wireless connection, such as by using any suitablewireless communications protocol known in the art. As such, theprocessor 204 may be configured to receive one or more signals from thesensors 252, 254, 256, 258.

As used herein, the term “processor” refers not only to integratedcircuits referred to in the art as being included in a computer, butalso refers to a controller, a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit, and other programmable circuits. The processor 204 is alsoconfigured to compute advanced control algorithms and communicate to avariety of Ethernet or serial-based protocols (Modbus, OPC, CAN, etc.).Additionally, the memory device(s) 207 may generally comprise memoryelement(s) including, but not limited to, computer readable medium(e.g., random access memory (RAM)), computer readable non-volatilemedium (e.g., a flash memory), a floppy disk, a compact disc-read onlymemory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc(DVD) and/or other suitable memory elements. Such memory device(s) 207may generally be configured to store suitable computer-readableinstructions that, when implemented by the processor(s) 204, configurethe controller 202 to perform the various functions as described herein.

Referring back to FIG. 2, the generator stator 120 may be electricallycoupled to a stator synchronizing switch 206 via the stator bus 208. Inone embodiment, the generator rotor 122 may be electrically coupled to abi-directional power converter assembly 210 or power converter via arotor bus 212. Alternatively, the generator rotor 122 may beelectrically coupled to the rotor bus 212 via any other device thatfacilitates operation of electrical power system 200 as describedherein. In a further embodiment, the stator synchronizing switch 206 maybe electrically coupled to the main transformer circuit breaker 214 viathe system bus 216.

In addition, as shown, the power converter assembly 210 (also referredto herein as a power converter) may include a rotor-side power converter220 electrically coupled to a line-side power converter 222 via a singledirect current (DC) link 244. Alternatively, the rotor-side powerconverter 220 and the line-side power converter 222 may be electricallycoupled via individual and separate DC links. In addition, as shown, theDC link 244 may include a positive rail 246, a negative rail 248, and atleast one capacitor 250 coupled therebetween.

In addition, the line-side power converter 222 may be electricallycoupled to a line bus 224 that includes a line contactor 226. Inaddition, the line contactor 226 may be electrically coupled to aconversion circuit breaker 228 via a conversion circuit breaker bus 230.In addition, the conversion circuit breaker 228 may be electricallycoupled to the main transformer circuit breaker 214 via system bus 216and a connection bus 232. The main transformer circuit breaker 214 maybe electrically coupled to an electric power main transformer 234 via agenerator-side bus 236. The main transformer 234 may be electricallycoupled to a grid circuit breaker 238 via the breaker-side bus 240. Inaddition, as shown, the grid circuit breaker 238 may be connected to theelectric power transmission and distribution grid via the grid bus 242.

In operation, alternating current (AC) power generated at the generatorstator 120 by rotation of the rotor 106 is provided via a dual path tothe grid bus 242. The dual paths are defined by the stator bus 208 andthe rotor bus 212. On the rotor bus side 212, sinusoidal multi-phase(e.g. three-phase) AC power is provided to the power converter assembly210. The rotor-side power converter 220 converts the AC power providedfrom the rotor bus 212 into DC power and provides the DC power to the DClink 244. Switching elements (e.g. IGBTs) used in bridge circuits of therotor-side power converter 220 can be modulated to convert the AC powerprovided from the rotor bus 212 into DC power suitable for the DC link244. In addition, as shown, the power converter assembly 210 may alsoinclude a dynamic brake assembly 260 electrically coupled between therotor-side converter 220 and the line-side converter 222, which will bediscussed in more detail below in reference to FIGS. 4-11.

Still referring to FIG. 2, the line-side converter 222 converts the DCpower on the DC link 244 into AC output power suitable for theelectrical grid bus 242. In particular, switching elements (e.g. IGBTs)used in bridge circuits of the line-side power converter 222 can bemodulated to convert the DC power on the DC link 244 into AC power onthe line-side bus 225. The AC power from the power converter assembly210 can be combined with the power from the generator stator 120 toprovide multi-phase power (e.g. three-phase power) having a frequencymaintained substantially at the frequency of the electrical grid bus 242(e.g. 50 Hz/60 Hz). It should be understood that the rotor-side powerconverter 220 and the line-side power converter 222 may have anyconfiguration using any switching devices that facilitate operation ofelectrical power system 200 as described herein.

Further, the power converter assembly 210 may be coupled in electronicdata communication with the turbine controller 202 and/or a separate orintegral converter controller 262 to control the operation of therotor-side power converter 220 and the line-side power converter 222.For example, during operation, one or more of the controllers 202, 262may be configured to receive one or more measurement signals from thesensors 252, 254, 256, 258. Thus, the controllers 202, 262 may beconfigured to monitor and control at least some of the operationalvariables associated with the wind turbine 100 via the sensors 252, 254,256, 258. In the illustrated embodiment, each of the sensors 252, 254,256, 258 may be electrically coupled to each one of the three phases ofthe power grid bus 242. Alternatively, the sensors 252, 254, 256, 258may be electrically coupled to any portion of electrical power system200, such as the DC link 244, that facilitates operation of electricalpower system 200 as described herein.

It should also be understood that any number or type of sensors may beemployed within the wind turbine 100 and at any location. For example,the sensors 252, 254, 256, 258 may be current or voltage transformers,shunt sensors, rogowski coils, Hall Effect current or voltage sensors,Micro Inertial Measurement Units (MIMUs), and/or any other suitablevoltage or electric current sensors now known or later developed in theart. Thus, the converter controller 262 is configured to receive one ormore feedback signals from the sensors 252, 254, 256, 258. In addition,the converter controller 262 may be configured with any of the featuresdescribed herein in regards to the main controller 202. Further, theconverter controller 262 may be separate from or integral with the maincontroller 202. As such, the converter controller 262 is configured toimplement the various method steps as described herein and may beconfigured similar to the turbine controller 202.

Referring now to FIGS. 4-11, schematic diagrams of various embodimentsof the dynamic brake assembly 260 according to the present disclosureare illustrated. More specifically, as shown, the dynamic brake assembly260 is electrically coupled to the DC link 244 between the positive andnegative rails 246, 248. Though the figures generally illustrate thedynamic brake assembly 260 connected to the positive rail 246, it shouldbe understood that the dynamic brake assembly 260 may also be coupled tothe negative rail 248. Further, as shown, the dynamic brake assembly 260includes a plurality of switching devices 264 connected in parallel. Forexample, as shown in the illustrated embodiments, the switching devices264 are insulated-gate bipolar transistors (IGBTs). In additionalembodiments, the switching devices 264 may also include one or morediodes. In addition, as shown, the dynamic brake assembly 260 includes aplurality of inductors 266 electrically coupled between the switchingdevices 264. In addition, as shown in FIGS. 4 and 5, the dynamic brakeassembly 260 may include at least one resistor 268 electrically coupledto a node 270 positioned between the inductors 266.

As shown particularly in FIG. 4, the plurality of switching devices 264may be arranged in a plurality of pairs 272 of switching devices 264connected in parallel. In such embodiments, as shown, each of theplurality of inductors 266 may be connected to nodes 274 between firstand second switching devices 264 of each of the plurality of pairs ofswitching devices 272. Alternatively, as shown in FIG. 5, each of theplurality of inductors 266 may be connected to nodes 276 associated withseparate switching devices 264 that are connected in parallel. Inaddition, as shown in FIG. 5, the dynamic brake assembly 260 may alsoinclude a freewheel diode 275 connected in series with each of theseparate switching devices 264. Further, each of the plurality ofinductors 266 may be connected in parallel with the resistor(s) 268.

Referring now to FIGS. 6-8, the dynamic brake assembly 260 may include aplurality of resistors 268. In such embodiments, as shown particularlyin FIG. 6, each of the plurality of resistors 268 may be connected inseries with one of the plurality of inductors 266 between the switchingdevices 264 to form a plurality of dynamic brake circuits 278. Thus, asshown in FIGS. 6-8, the dynamic brake assembly 260 may further includeat least one snubber capacitor 280 electrically coupled between theplurality of dynamic brake circuits 278. In such embodiments, theaddition of the snubber capacitor 280 between the two dynamic brakecircuits is configured to change the operation of the circuit byallowing significant reduction of the turn-off switching loss. Forexample, in the illustrated circuit, the turn-on of the switchingdevices 264 can be simultaneous, whereas turn-off can be staggered, suchthat one switching device turns off before the other.

In addition, as shown in FIG. 7, the dynamic brake assembly 260 may alsoinclude at least one additional resistor 282 connected in series withthe snubber capacitor 280. More specifically, as shown, the dynamicbrake assembly 260 includes two additional resistors 282 connected inseries on opposing sides of the snubber capacitor 280. In anotherembodiment, the dynamic brake assembly 260 may include at least oneadditional resistor 282 connected in parallel with the snubber capacitor280. More specifically, as shown in FIG. 8, the dynamic brake assembly260 includes two additional resistors 282 connected in parallel with thesnubber capacitor 280.

Referring now to FIGS. 9 and 10, the resistor(s) 268 of the dynamicbrake assembly 260 may include a split resistor 284, i.e. one thatdivides voltage between multiple routes. For example, as shownparticularly in FIG. 9, at least one inductor 266 is connected to eachconnection end of the split resistor 284 to the dynamic brake phasemodule. In addition, as shown in FIG. 9, such inductors 266 are separatecomponents in the circuit. In an alternative embodiment, as shown inFIG. 10, the dynamic brake assembly 260 includes a plurality ofswitching devices 264 connected in parallel with at least oneresistance-inductance component 286 electrically coupled between theplurality of switching devices 264. In such embodiments, theresistance-inductance component 286 is a single component having bothresistance 288 and inductance 290 capabilities, i.e. in separatesections of the component.

Referring now to FIG. 11, a schematic diagram of yet another embodimentof the dynamic brake assembly 260 according to the present disclosure isillustrated. As shown, the dynamic brake assembly 260 includes aplurality of switching devices 264 connected in parallel with aninductance component 292 electrically coupled between the switchingdevices 264. In such embodiments, the inductance component 292 is asingle component having inductance capabilities that also has specifiedand controlled resistance. In such embodiments, the inductance andresistance are distributed in the composition of the component.

Referring now to FIG. 12, a flow diagram of one embodiment of a method300 for controlling peak voltage of a DC link of a power converter of anelectrical power system connected to a power grid with minimal switchinglosses is illustrated. In general, the method 300 will be describedherein with reference to the wind turbine 100 and dynamic brake assembly260 shown in FIGS. 1-11. However, it should be appreciated that thedisclosed method 300 may be implemented with wind turbines having anyother suitable configurations. In addition, although FIG. 12 depictssteps performed in a particular order for purposes of illustration anddiscussion, the methods discussed herein are not limited to anyparticular order or arrangement. One skilled in the art, using thedisclosures provided herein, will appreciate that various steps of themethods disclosed herein can be omitted, rearranged, combined, and/oradapted in various ways without deviating from the scope of the presentdisclosure.

As shown at (302), the method 300 includes electrically coupling thedynamic brake assembly 260 (such as any of the embodiments illustratedin FIGS. 4-11, to the DC link 244 of the power converter 210. As shownat (304), the method 300 includes receiving a voltage measurement of theDC link 244 of the power converter 210. For example, as shown in FIG.13, a schematic diagram of one embodiment of a control scheme that maybe implemented by one of the controllers described herein 202, 262 isillustrated. As shown, the controller 202, 262 receives the voltagemeasurement 294 via comparator 296. Referring back to FIG. 12, as shownat (312), the method 300 may also include applying hysteresis to thevoltage measurement 294.

In addition, as shown at (306), the method 300 further includescomparing the voltage measurement 294 of the DC link 244 to a voltagethreshold 298. For example, as shown in FIG. 13, the comparator 296 isconfigured to compare the voltage measurement 294 and the voltagereference 298 or threshold. Referring back to FIG. 12, as shown at(308), the controller 202, 262 is configured to determine whether thevoltage measurement 294 is equal to or exceeds the voltage threshold298. If so, as shown at (310), the method 300 includes turning on thedynamic brake assembly 260 of the power converter 210 such that theinductor(s) 266 receives at least part of a load generated by the powerconverter 210. More specifically, as shown in the illustrated embodimentof FIG. 13, the controller 202 may send an “ON” signal 295 to theconverter controller 262 such that the converter controller 262 can sendappropriate gating commands to the switching devices 264 (i.e. IGBT 1and IGBT 2). More specifically, in certain embodiments, the controller202, 262 may alternate on-delays and off-delays of the switching devices264 to optimize sharing of a load between the switching devices 264.

In further embodiments, such as those that include the snubber capacitor280, the controller 202, 262 may simultaneously turn on the switchingdevices 264 to optimize sharing of a load between the switching devices264. As such, there are minimal to no switching losses at turn on. Beinga primarily resistive load, the load current is zero when the switchingdevices 264 turn on, allowing a zero-current turn-on with no switchingloss, as long as both switching devices 264 can be previously turned offlong enough to allow the load current to drop to zero.

In addition, the controller 202, 262 may stagger turn-off commands forthe switching devices 264 such that each switching device turns off at adifferent time. As such, the first switching device 264 that turns offwill have zero (or much reduced) switching loss because the load currentwill shift into the capacitor 280 during the switching event. Hence, thecapacitor 280 becomes a turn-off snubber circuit. The second switchingdevice 264 that turns off will have reduced switching loss (nearly zero)because the capacitor 280 will again act as a turn-off snubber. It maybe advantageous, but not necessary, to operate the turn-off of theswitching devices 264 in a way to alternate the sequence for every otherturn-off event, (one switching device 264 turns off first, then theother), to better balance their switching, conduction and diode losses.

Referring now to FIG. 14, a schematic diagram of one embodiment of adetailed control scheme 350 for the dynamic brake assembly 260 describedherein according to the present disclosure is illustrated. As shown, adynamic brake (DB) voltage regulator 352 (such as aproportional-integral regulator) receives a maximum DC link limit value(e.g. a fixed voltage threshold) and an actual DC link voltage feedbacksignal (i.e. a variable value). The voltage regulator 352 then generatesan output signal 354 that can be received and used by a plurality ofdynamic brake modulators 356, 358 for generating respective gatingcommands 360, 362 for first and second switching devices 264 (e.g. IGBT1 and IGBT 2).

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A power converter assembly for an electricalpower system connected to a power grid, the power converter assemblycomprising: a rotor-side converter configured for coupling to agenerator rotor of a generator of the electrical power system; aline-side converter electrically coupled to rotor-side converter via aDC link, the line-side converter configured for coupling to the powergrid; at least one sensor configured to monitor at voltage parameter ofthe DC link; and, a dynamic brake assembly electrically coupled to theDC link, the dynamic brake assembly comprising a plurality of switchingdevices connected in parallel and a plurality of inductors electricallycoupled between the plurality of switching devices, wherein, when thevoltage parameter is at or above a voltage threshold, the dynamic brakeassembly is configured to turn on such that the plurality of inductorsreceives at least part of a load generated by the power converterassembly.
 2. The power converter assembly of claim 1, wherein thedynamic brake assembly further comprises a resistor electrically coupledto a node positioned between the plurality of inductors.
 3. The powerconverter assembly of claim 2, wherein the resistor comprises a splitresistor.
 4. The power converter assembly of claim 2, wherein theplurality of switching devices may be arranged in a plurality of pairsof switching devices connected in parallel.
 5. The power converterassembly of claim 4, wherein each of the plurality of inductors areconnected to nodes between first and second switching devices of each ofthe plurality of pairs of switching devices.
 6. The power converterassembly of claim 2, wherein each of the plurality of inductors isconnected in parallel with the resistor.
 7. The power converter assemblyof claim 1, wherein the dynamic brake assembly further comprises aplurality of resistors, each of the plurality of resistors beingconnected in series with one of the plurality of inductors between theplurality of switching devices to form a plurality of dynamic brakecircuits.
 8. The power converter assembly of claim 7, wherein thedynamic brake assembly further comprises at least one snubber capacitorelectrically coupled between the plurality of dynamic brake circuits. 9.The power converter assembly of claim 8, wherein the dynamic brakeassembly further comprises at least one additional resistor connected inseries with the snubber capacitor.
 10. The power converter assembly ofclaim 9, wherein the dynamic brake assembly further comprises at leastone additional resistor connected in parallel with the snubbercapacitor.
 11. The power converter assembly of claim 1, wherein theplurality of switching devices comprise insulated-gate bipolartransistors (IGBTs).
 12. The power converter assembly of claim 1,wherein the electrical power system is part of a wind turbine powersystem, and the generator comprises a doubly-fed induction generator(DFIG).
 13. A power converter assembly for an electrical power systemconnected to a power grid, the power converter assembly comprising: arotor-side converter configured for coupling to a generator rotor of agenerator of the electrical power system; a line-side converterelectrically coupled to rotor-side converter via a DC link, theline-side converter configured for coupling to the power grid; at leastone sensor configured to monitor at voltage parameter of the DC link;and, a dynamic brake assembly electrically coupled to the DC link, thedynamic brake assembly comprising a plurality of switching devicesconnected in parallel and at least one resistance-inductance componentelectrically coupled between the plurality of switching devices,wherein, when the voltage parameter is at or above a voltage threshold,the dynamic brake assembly is configured to turn on such that the atleast one resistance-inductance component receives at least part of aload generated by the power converter assembly.
 14. A method forcontrolling peak voltage of a DC link of a power converter of anelectrical power system connected to a power grid with minimal switchinglosses, the method comprising: electrically coupling a dynamic brakeassembly to a DC link of the power converter, the dynamic brake having aplurality of switching devices connected in parallel and a plurality ofinductors electrically coupled between the plurality of switchingdevices; receiving a voltage measurement of the DC link of the powerconverter; comparing the voltage measurement of the DC link to a voltagethreshold; when the voltage measurement is at or above the voltagethreshold, turning on the dynamic brake assembly of the power convertersuch that the at least one inductor receives at least part of a loadgenerated by the power converter.
 15. The method of claim 14, furthercomprising applying hysteresis to the voltage measurement.
 16. Themethod of claim 14, wherein turning on the dynamic brake assembly of thepower converter further comprises determining at least one gatingcommand for each of the plurality of switching devices.
 17. The methodof claim 16, wherein determining the at least one gating command foreach of the plurality of switching devices further comprisestime-shifting on-delays and off-delays of the plurality of switchingdevices to optimize sharing of a load between the plurality of switchingdevices.
 18. The method of claim 14, wherein the dynamic brake assemblyfurther comprises a plurality of resistors and a plurality of inductorscoupled between the plurality of switching devices, each of theplurality of resistors connected in series with one of the plurality ofinductors to form a plurality of dynamic brake circuits, the dynamicbrake assembly further comprising at least one snubber capacitor coupledbetween the plurality of dynamic brake circuits.
 19. The method of claim18, wherein determining the at least one gating command for each of theplurality of switching devices further comprises: simultaneously turningon the plurality of switching devices to optimize sharing of a loadbetween the plurality of switching devices; and, time-shifting turn-offcommands for the plurality of switching devices such that each switchingdevice turns off at a different time.
 20. The method of claim 14,wherein the dynamic brake assembly further comprises a resistorelectrically coupled to a node positioned between the plurality ofinductors.